Protein kinases participate in the signaling events which control the activation, growth and differentiation of cells in response to extracellular mediators and to changes in the environment. In general, these kinases fall into two groups; those which preferentially phosphorylate serine and/or threonine residues and those which preferentially phosphorylate tyrosine residues (Hanks & Hunter T, 1995, FASEB. J. 9:576-596). The serine/threonine kinases include, for example, protein kinase C isoforms (Newton, 1995, J. Biol. Chem. 270:28495-28498) and a group of cyclin-dependent kinases such as cdc2 (Pines, 1995, Trends in Biochemical Sciences 18:195-197). The tyrosine kinases include membrane-spanning growth factor receptors such as the epidermal growth factor receptor (Iwashita & Kobayashi, 1992, Cellular Signaling 4:123-132), and cytosolic non-receptor kinases such as ZAP-70 and csk kinases (Chan et al., 1994, Ann. Rev. Immunol. 12:555-592).
Inappropriately high protein kinase activity has been implicated in many diseases resulting from abnormal cellular function. This might arise either directly or indirectly, for example by failure of the proper control mechanisms for the kinase, related for example to mutation, over-expression or inappropriate activation of the enzyme; or by over- or underproduction of cytokines or growth factors also participating in the transduction of signal upstream or downstream of the kinase. In all of these instances, selective inhibition of the action of the kinase might be expected to have a beneficial effect.
All of the protein kinases that have been identified to date in the human genome share a highly conserved catalytic domain of around 300 aa. This domain folds into a bi-lobed structure in which reside ATP-binding and catalytic sites. The complexity of protein kinase regulation allows many potential mechanisms of inhibition including competition with activating ligands, modulation of positive and negative regulators, interference with protein dimerization, and allosteric or competitive inhibition at the substrate or ATP binding sites.
2.1 Axl Kinase
Axl (also known as UFO, ARK, and Tyrol; nucleotide accession numbers NM_021913 and NM_001699; protein accession numbers NP_068713 and NP_001690) is a receptor protein tyrosine kinase (RTK) that comprises a C-terminal extracellular ligand-binding domain and N-terminal cytoplasmic region containing the catalytic domain. The extracellular domain of Axl has a unique structure that juxtaposes immunoglobulin and fibronectin Type III repeats and is reminiscent of the structure of neural cell adhesion molecules. Axl and its two close relatives, Mer/Nyk and Sky (Tyro3/Rse/Dtk), collectively known as the Tyro3 family of RTKs, all bind and are stimulated to varying degrees by the same ligand, Gas6 (growth arrest specific-6), a ˜76 kDa secreted protein with significant homology to the coagulation cascade regulator, Protein S. In addition to binding to ligands, the Axl extracellular domain has been shown to undergo homophilic interactions that mediate cell aggregation, suggesting that one important function of Axl may be to mediate cell-cell adhesion.
Axl is predominantly expressed in the vasculature in both endothelial cells (ECs) and vascular smooth muscle cells (VSMCs) and in cells of the myeloid lineage and is also detected in breast epithelial cells, chondrocytes, Sertoli cells and neurons. Several functions including protection from apoptosis induced by serum starvation, TNF-α or the viral protein E1A, as well as migration and cell differentiation have been ascribed to Axl signaling in cell culture. However, Axl−/− mice exhibit no overt developmental phenotype and the physiological function of Axl in vivo is not clearly established in the literature.
Angiogenesis (the formation of new blood vessels) is limited to functions such as wound healing and the female reproductive cycle in healthy adults. This physiological process has been co-opted by tumors, thus securing an adequate blood supply that feeds tumor growth and facilitates metastasis. Deregulated angiogenesis is also a feature of many other diseases (for example, psoriasis, rheumatoid arthritis, endometriosis and blindness due to age-related macular degeneration (AMD), retinopathy of prematurity and diabetes) and often contributes to the progression or pathology of the condition.
The overexpression of Axl and/or its ligand has also been reported in a wide variety of solid tumor types including, but not limited to, breast, renal, endometrial, ovarian, thyroid, non-small cell lung carcinoma, and uveal melanoma as well as in myeloid leukemias. Furthermore, it possesses transforming activity in NIH3T3 and 32D cells. It has been demonstrated that loss of Axl expression in tumor cells blocks the growth of solid human neoplasms in an in vivo MDA-MB-231 breast carcinoma xenograft model. Taken together, these data suggest Axl signaling can independently regulate EC angiogenesis and tumor growth and thus represents a novel target class for tumor therapeutic development.
The expression of Axl and Gas6 proteins is upregulated in a variety of other disease states including endometriosis, vascular injury and kidney disease and Axl signaling is functionally implicated in the latter two indications. Axl-Gas6 signaling amplifies platelet responses and is implicated in thrombus formation. Axl may thus potentially represent a therapeutic target for a number of diverse pathological conditions including solid tumors, including, but not limited to, breast, renal, endometrial, ovarian, thyroid, non-small cell lung carcinoma and uveal melanoma; liquid tumors, including but not limited to, leukemias (particularly myeloid leukemias) and lymphomas; endometriosis, vascular disease/injury (including but not limited to restenosis, atherosclerosis and thrombosis), psoriasis; visual impairment due to macular degeneration; diabetic retinopathy and retinopathy of prematurity; kidney disease (including but not limited to glomerulonephritis, diabetic nephropathy and renal transplant rejection), rheumatoid arthritis; osteoarthritis and cataracts.
2.2 JAK Kinase
JAK kinases (JAnus Kinases) are a family of cytoplasmic protein tyrosine kinases including JAK1, JAK2, JAK3 and TYK2. Each of the JAK kinases is selective for the receptors of certain cytokines, though multiple JAK kinases may be affected by particular cytokine or signaling pathways. Studies suggest that JAK3 associates with the common gamma (γc) chain of the various cytokine receptors. JAK3 in particular selectively binds to receptors and is part of the cytokine signaling pathway for IL-2, IL-4, IL-7, IL-9, IL-15 and IL-21. JAK1 interacts with, among others, the receptors for cytokines IL-2, IL-4, IL-7, IL-9 and IL-21, while JAK2 interacts with, among others, the receptors for IL-9 and TNF-α. Upon binding of certain cytokines to their receptors (e.g., IL-2, IL-4, IL-7, IL-9, IL-15 and IL-21), receptor oligomerization occurs, resulting in the cytoplasmic tails of associated JAK kinases being brought into proximity and facilitating the trans-phosphorylation of tyrosine residues on the JAK kinase. This trans-phosphorylation results in the activation of the JAK kinase.
Phosphorylated JAK kinases bind various STAT (Signal Transducer and Activator of Transcription) proteins. STAT proteins, which are DNA binding proteins activated by phosphorylation of tyrosine residues, function both as signaling molecules and transcription factors and ultimately bind to specific DNA sequences present in the promoters of cytokine-responsive genes (Leonard et al., (2000), J. Allergy Clin. Immunol. 105:877-888). JAK/STAT signaling has been implicated in the mediation of many abnormal immune responses such as allergies, asthma, autoimmune diseases such as transplant (allograft) rejection, rheumatoid arthritis, amyotrophic lateral sclerosis and multiple sclerosis, as well as in solid and hematologic malignancies such as leukemia and lymphomas. For a review of the pharmaceutical intervention of the JAK/STAT pathway see Frank, (1999), Mol. Med. 5:432:456 and Seidel et al., (2000), Oncogene 19:2645-2656.
JAK3 in particular has been implicated in a variety of biological processes. For example, the proliferation and survival of murine mast cells induced by IL-4 and IL-9 have been shown to be dependent on JAK3- and gamma chain-signaling (Suzuki et al., (2000), Blood 96:2172-2180). JAK3 also plays a crucial role in IgE receptor-mediated mast cell degranulation responses (Malaviya et al., (1999), Biochem. Biophys. Res. Commun. 257:807-813), and inhibition of JAK3 kinase has been shown to prevent type I hypersensitivity reactions, including anaphylaxis (Malaviya et al., (1999), J. Biol. Chem. 274:27028-27038). JAK3 inhibition has also been shown to result in immune suppression for allograft rejection (Kirken, (2001), Transpl. Proc. 33:3268-3270). JAK3 kinases have also been implicated in the mechanism involved in early and late stages of rheumatoid arthritis (Muller-Ladner et al., (2000), J. Immunal. 164:3894-3901); familial amyotrophic lateral sclerosis (Trieu et al., (2000), Biochem Biophys. Res. Commun. 267:22-25); leukemia (Sudbeck et al., (1999), Clin. Cancer Res. 5:1569-1582); mycosis fungoides, a form of T-cell lymphoma (Nielsen et al., (1997), Prac. Natl. Acad. Sci. USA 94:6764-6769); and abnormal cell growth (Yu et al., (1997), J. Immunol. 159:5206-5210; Catlett-Falcone et al., (1999), Immunity 10:105-115).
The JAK kinases, including JAK3, are abundantly expressed in primary leukemic cells from children with acute lymphoblastic leukemia, the most common form of childhood cancer, and studies have correlated STAT activation in certain cells with signals regulating apoptosis (Demoulin et al., (1996), Mol. Cell. Biol. 16:4710-6; Jurlander et al., (1997), Blood. 89:4146-52; Kaneko et al., (1997), Clin. Exp. Immun. 109:185-193; and Nakamura et al., (1996), J. Biol. Chem. 271:19483-8). They are also known to be important to lymphocyte differentiation, function and survival. JAK-3 in particular plays an essential role in the function of lymphocytes, macrophages, and mast cells. Given the importance of this JAK kinase, compounds which modulate the JAK pathway, including those selective for JAK3, can be useful for treating diseases or conditions where the function of lymphocytes, macrophages, or mast cells is involved (Kudlacz et al., (2004) Am. J. Transplant 4:51-57; Changelian (2003) Science 302:875-878). Conditions in which targeting of the JAK pathway or modulation of the JAK kinases, particularly JAK3, may be therapeutically useful include, leukemia, lymphoma, transplant rejection (e.g. pancreas islet transplant rejection, bone marrow transplant applications (e.g. graft-versus-host disease), autoimmune diseases (e.g. diabetes), and inflammation (e.g. asthma, allergic reactions). Conditions which may benefit for inhibition of JAK3 are discussed in greater detail below.
In view of the numerous conditions that may benefit by treatment involving modulation of the JAK pathway it is immediately apparent that new compounds that modulate JAK pathways and methods of using these compounds should provide substantial therapeutic benefit to a wide variety of patients.
2.3 SYK Kinase
Crosslinking of Fc receptors, such as the high affinity receptor for IgE (FcεRI) and/or the high affinity receptor for IgG (FcγRI) activates a signaling cascade in mast, basophil and other immune cells that results in the release of chemical mediators responsible for numerous adverse events. For example, such crosslinking leads to the release of preformed mediators of Type I (immediate) anaphylactic hypersensitivity reactions, such as histamine, from storage sites in granules via degranulation. It also leads to the synthesis and release of other mediators, including leukotrienes, prostaglandins and platelet-activating factors (PAFs), that play important roles in inflammatory reactions. Additional mediators that are synthesized and released upon crosslinking Fc receptors include cytokines and nitric oxide.
The signaling cascade(s) activated by crosslinking Fc receptors such as FcεRI and/or FcγRI comprises an array of cellular proteins. Among the most important intracellular signal propagators are the tyrosine kinases. And, an important tyrosine kinase involved in the signal transduction pathways associated with crosslinking the FcεRI and/or FcγRI receptors, as well as other signal transduction cascades, is Syk kinase (see Valent et al., 2002, Intl. J. Hematol. 75(4):257-362 for review). The mediators released as a result of FcεRI and FcγRI receptor cross-linking are responsible for, or play important roles in, the manifestation of numerous adverse events. Therefore, there exists a need for compounds which are able to effectively inhibit Syk kinase.